Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
9.2.3.2. Grouping
In order to simplify the study, all the initiating events that can be identified are grouped
together in a limited number of families having similar functional consequences.
In certain cases, these families are broken down into subfamilies, depending on the
functional consequences of the accident. In particular, initiating events considered in
different reactor states generally constitute separate sub-families.
It should also be noted that the families and sub-families thus defined are not
independent. Indeed, an initiating event may lead to accident sequences that result in an
event considered to be an initiator of another family. For example, primary system
overpressure transients can cause a pressurizer break, which constitutes a sub-initiator of
the primary break family. All such interaction between families must be taken into
account in quantification.
9.2.4. Accident sequences
9.2.4.1 Accident sequence modeling
Once initiating events have been identified and grouped, it is necessary to determine the
response of the plant to each group of initiating events. This modelling of responses
results in the generation of accident (or event) sequences.
The mostclassical method for the event sequences definition is the conventional event
tree method. For reference, this is a logical method consisting of taking an initiating event
and systematically envisaging the success or failure of all the systems and functions liable
to play a role during the accident situation. Each branch of the tree represents an accident
sequence which is then analysed to determine which branches lead to core damagedown
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